Field of the Invention
The invention concerns the determination of sensitivity profiles of local coils in magnetic resonance technology.
Description of the Prior Art
Magnetic resonance technique (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed in a simplified form, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (also called a B-field) with a field strength of 0.2 to 7 Tesla or more in a magnetic resonance apparatus, such nuclear spins in the subject orient along the basic magnetic field. To trigger magnetic resonances, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject, the triggered nuclear magnetic resonances are measured as what are known as k-space data, and MR images are reconstructed or spectroscopy data are determined on the basis of these. For spatial coding of the measurement data, rapidly activated magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a memory organized as a k-space matrix. For example, an associated MR image can be reconstructed by a multidimensional Fourier transformation of the data (values) in the k-space matrix.
The measurement of the magnetic resonances (i.e., the MR signal) takes place with at least one acquisition coil, wherein normally an antenna known as a “body coil” is integrated into the magnetic resonance apparatus. However, this body coil is relatively far removed from the subject to be examined (a patient, for example) due to the integration into the magnetic resonance apparatus. Therefore, smaller coils—known as local coils—are often also used to receive the magnetic resonance signals, which local coils can be placed directly on the subject to be examined, or at least near to the subject, in order to achieve a better signal-to-noise ratio.
Such local coils often embody a large number of smaller acquisition coils. Each coil element has a channel. By utilizing the multiple acquisition channels that are thus present, advanced imaging methods such as parallel imaging can be executed. The information of the individual channels is combined in the image reconstruction algorithm and presented in the MR image.
Depending on the alignment and spacing of the coil elements of the local coil from the measured subject, the individual channels of the acquisition coil acquire signals of differing strengths. When respective signals of the channel are combined, this can therefore lead to signal inhomogeneities in the MR image in which, for example, regions that are situated closer to the coils will appear markedly brighter than regions situated more distant (for example inside the subject).
In order to avoid these inhomogeneities, for every MR examination the (current) sensitivity distribution of the local coil is measured, most often in a preceding measurement. In this measurement, an image of the measured subject is acquired with the body coil and an image is acquired with the local coil that is being used. In comparison to the image acquired with the local coil, the image acquired with the body coil has a high homogeneity. By processing the two images (which includes a division, among other things), the intensity distribution of the local coil image (and therefore its sensitivity distribution) can be calculated. In the following (for example clinical) MR measurements, arising signal inhomogeneities can be remedied using this sensitivity distribution of the local coil, and a homogeneous image can be created.
A gradient echo sequence (GRE) is most often used as a measurement sequence for such upstream measurements to determine the sensitivity distribution of the local coil. The sequence that is used is very loud. Usually, noise levels of well over 90 dB(A) occur due to the gradients to be switched.